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  Java Practices -> Output parameters
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<div class='page-title'>Output parameters</div>

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<br>A method may <em>occasionally</em> need to use a parameter for its return value
- what might be loosely called an "output parameter" or a "result parameter".
The caller creates an output parameter object, and then passes it to a
method which changes its state. When the method returns, the caller then
examines this new state.
<p>Output parameters :
<ul>
<li>must be visible in both a method and its caller</li>
<li>cannot be <tt>null</tt>, since the object needs to be created by the caller</li>
<li>
cannot be <a href="TopicActiond838-2.html">immutable objects</a> - a <tt>String</tt>, for example, 
cannot be use as an output parameter, since it cannot change state</li>
<li>should probably be used only occasionally, and after careful consideration</li>
</ul>
<b>Example</b>
<p>Here, there is both an output parameter and a <tt>boolean</tt> return
value: if the output parameter is indeed populated, then the return value
is set to <tt>true</tt>.
<br>
<PRE>

<span class='keyword'>import</span> javax.servlet.http.*;

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> CookieValue {

   <span class='comment'>/**
   * Determine if a user's request contains a particular cookie, and if it does
   * then place its value in an "out" parameter.
   *
   * Example
   * &lt;pre&gt;
   * if ( CookieValue.getCookieValue("ID", aRequest, aIdValue) ) {
   *   //use aIdValue
   * }
   * &lt;/pre&gt;
   *
   * @param aCookieName the non-null name attached to the Cookie.
   * @param aRequest is the non-null request forwarded from the user's browser which
   * is to be examined for the presence of the given cookie.
   * @param aCookieValue is a non-null "out" parameter in which the
   * cookie's value is placed - if found - for further examination by the
   * caller; it must have a zero length.
   * @throws IllegalArgumentException if aCookieValue.length() &gt; 0.
   */</span>
   <span class='keyword'>static</span> <span class='keyword'>boolean</span> getCookieValue(
     String aCookieName, HttpServletRequest aRequest, <span class='keyword'>final</span> StringBuilder aCookieValue
   ){
      <span class='keyword'>if</span> (aCookieValue.length()&gt;<span class='literal'>0</span>) <span class='keyword'>throw</span> <span class='keyword'>new</span> IllegalArgumentException();
      <span class='keyword'>boolean</span> userHasCookie = <span class='keyword'>false</span>;
      Cookie[] cookies = aRequest.getCookies();
      <span class='keyword'>if</span> (cookies != <span class='keyword'>null</span>){
        <span class='keyword'>for</span>(Cookie cookie : cookies){
          <span class='keyword'>if</span> ( aCookieName.equals(cookie.getName())){          
            userHasCookie = <span class='keyword'>true</span>;
            <span class='comment'>//change the state of the output param (aCookieValue)
</span>            aCookieValue.append(cookie.getValue());
          }
        }
      }
      <span class='keyword'>return</span> userHasCookie;
   }
} 
</PRE>
<br>
<br>An alternative design is to return a <tt>null</tt> value if the cookie
is not actually present. This would replace the current <tt>boolean</tt>
return value with a <tt>String</tt> return value which should be checked
for <tt>null</tt>. However, that alternative design seems weaker, since 
it seems prudent to <a href='TopicAction772f-2.html'>avoid using null</a> 
return values.
<br>
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<div class='topic-section'>See Also :</div>
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  <a href='TopicActiond838-2.html?Id=29'>Immutable objects</a> <br>
 
  
  <a href='TopicAction772f-2.html?Id=134'>Avoid null if possible</a> <br>
 
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